When sculpting, my brush changes from Add to Subtract and viceversa
If you try to enter Sculpt Mode with your model, you can see the warning message Object has non-uniform scale, sculpting may be inpredictable .




First, select the object in Object Mode and then Apply the Scale by Ctrl+A > Scale.
Then go into Edit Mode, select all by A and Make Normals Consistent by pressing Ctrl+N.
If normals are now inverted, invert them back by W > Flip Normals.


Then your sculpting tool should work equally on the whole model.